Refining of liquid hydrocarbons and fractionation provides a series of streams of hydrocarbon products. A stream such as hydrocarbon feed, unhydrotreated pyrolysis gasoline from steam cracker, FCC naphtha or unhydrotreated coker naphtha is further refined to provide a mixture of C6-C8 aromatics, commonly referred to as BTX, comprising primarily benzene, toluene, ethyl benzene, styrene, sulphur compounds and a mixture of xylenes. For example, a process for production of BTX from FCC naphtha is described by Timken et al. in U.S. Pat. No. 5,685,972 issued in 1997.
BTX is a valuable feedstock for manufacture of petrochemicals and polymers, and is also used as fuel for internal combustion engines. However, its styrene content tends to polymerize and form higher molecular weight compounds that can interfere with processing of BTX as chemical feedstock, or can cause formation of gummy residue that interferes with feeding it for combustion. Hence the presence of styrene in BTX is undesirable when BTX is to be used as petrochemical feedstock or as a liquid fuel for internal combustion engines. The styrene content in BTX is reduced by conversion to ethyl benzene by hydrogenation. Timken et al. in U.S. Pat. No. 5,685,972 describe a “hydrofinishing” stage in conversion of FCC naphtha to both BTX and high octane gasoline.
However, ethylbenzene and styrene have low value when they are combusted as fuel. BTX from which styrene has been removed has higher value than BTX containing styrene. Further, styrene itself has much higher value when it is recovered for use in manufacture of polymers or petrochemicals when compared to its conversion to ethyl benzene for use as fuel. So far, none of the methods disclosed in, for example, U.S. Pat. Nos. 3,953,300, 4,031,153, and 5,849,982, are effective for removing and recovering styrene from BTX fractions such as pyrolysis gasoline and FCC gasoline, which contain a significant amount of sulphur compounds. This is because styrene is more reactive with hydrogen than thiophenic sulfur in hydrotreating which is the only way to desulfurize the purified styrene stream commercially.
What is required is a process that removes styrene from BTX fractions more efficiently than present BTX refining and separation processes, and converting that styrene to valuable polymers and petrochemical products.